Current transformer



June 20, 1933.

A. o. AUSTIN CURRENT TRANSFORMER Filed Nov. 17, 1930 3 Sheets-Sheet 1 BY %AZLOKJ A TTORNEY June 20, 1933. A. o. AUSTIN 7 1,914,395

CURRENT TRANSFORMER Filed Nov. 17, 1930 I 3 Sheets-Sheet 2 INVENTOR Byway 6 A TTORNE K.

June 20, 1933. A. o. AUSTIN 1,914,395

CURRENT TRANSFORMER Filed Nov. 17, 1930 3 Sheets-Sheet 3 y BY E M A TTOR/VE Y Patented June 20, 1933 UNITED STATES PATENT OFFICE ARTHUR O. AUSTIN, OF NEAR BARBERTON, OHIO, ASSIGNOR, BY MESNE- ASSIGNMENTS, TO THE OHIO BRASS COMPANY, OF MANSFIELD, OHIO, 1 CORPORATION OF NEW JERSEY CURRENT TRANSFORMER Application filed November 1'7, 1930. Serial No. 496,083.

This invention relates to current transformers and has for one of its objects the provision of a current transformer arrangement which shall obviate difiiculties hereto- 8 fore experienced for various burdens on the secondary of the transformer.

Another object is to provide a method of operating current transformers by means of which the characteristics of the burden on the 10 secondary of the transformer will be maintained constant.

Other objects and advantages will appear from the following description. p

The invention is exemplified by the combination and arrangement of parts shown 1n the accompanying drawings and described in the following specification, and it is more particularly pointed out in the appended claims.

In the drawings:

F ig. 1 is a diagrammatic, vertical section of a bushing insulator showing one application of the present invention.

Figs. 2, 3, 4, 6, 8, 9, 10 and 14 are diagrams showing other applications of the invention.

Fig. 5 is a vector diagram illustrating the operation of the invention shown in Fig. 1.

Fig. 7 is a vector diagram illustrating the relation of different burdens which may constitute the total burden on the secondary of the transformer.

Fig. 11 is a vector diagram of currents and voltages in the circuits containing the useful burdens in Fig. 10.

Fig. 12 is a Vector diagram of the currents in the parallel circuits containing the useful burdens of Fig. 10.

Fig. 13 is a vector diagram of the voltages in the series burdens of Fig. 10.

Fig. 15 is a vector diagram of voltages and current in the series circuit of Fig. 14.

Fig. 16 is a vector diagram of the currents in Fig. 14. v

Fig. 17 is a vector diagram showing the 45 relation of current and voltage of Fig. 14.

i In the use of bushing and other types of current transformers where the primary consists of a single lead only, it is frequently diflicult to obtain satisfactory characteristics 0 in the transformer and the necessary energy for eflicient operation of relays, Watt-meters, watt-hour meters, ammeters or other equipment. In my prior Patent No. 1,723,000 I have shown a type of current transformer which may be used to advantage in supplying the necessary voltampere output, even though the current in the primary is small.

In my Patent No. 1,728,170 and applications Serial Numbers339,001, 400,267, 406,284 and 433,921, I have shown methods of correcting the phase angle and ratio over a wide range of current in the primary and for a change in duty in the secondary. In the ordinary type of bushing transformers or those employing a single lead only, for the primary, it has been customary to use a ringshaped magnetic core on which the secondary of the transformer is wound. The secondaries of these transformers are used to operate individual apparatus that maybe connected in multiple or in series. Where they are applied to high voltage bushings, iowever, they cover a very considerable portion of the insulation of the bushing, thereby reducing its factor of safety or making it necessary to increase the length and consequently the cost of the bushing. The ring type transformer placed over the outside of the bushing will, of necessity, have a rather long magnetic path. The characteristics of this transformer, both as regards phase angle and output, are very poor, particularly for small currents in the primary.

With the form of transformer disclosed in my prior patents cited above, it is possible to place the secondary of the current transformer inside the bushing or in an insulating pocket outside the bushing so that the length of the bushing need not be increased for a given amount of insulation.

. W'here the available output of the secondaryof the current transformer is sufiicient, it is possible to use a single current transformerfor all requirements, correcting the phase angle and ratio for variations in the load by means shown in my prior patents and applications mentioned above. Where the load or burden of the secondary is changed, however, some means of compensation will be necessary in order that conditions will not be disturbed for the various burdens connected to the secondary. WVith the present invention it is possible to maintain constant load conditions even though several burdens are connected, either separately or in various combinations, to the secondary of the current transformer. The method also makes it possible to reduce the number of taps to the secondary and, at the same time, obtain the desired ratios.

In Fig. 1 is shown a current transformer having a primary lead 10 with a winding 11 around a magnetic core 12. The primary winding has a shunt 13 with a reactance 14 and a resistance 15, together with a series impedance 16. The use of a reactance in the primary shunt tends to advance the phase angle in the secondary. Therefore, the use of a capacitance 55, used as a shunt, will change the phase angle in the opposite direction. Since the voltages are very small, it may be advisable to raise the voltage on the condenser through a transformer 56 which increases the potential on the condenser 57. It is understood that any combination of any impedances and any characteristics in the shunts, either as to phase angle or ratio, may be used to obtain the desired results.

It is further understood that these may be varied in accordance with the current by suitable relays as disclosed in my previous applications referred to above. These relays may be used in either the primary or secondary circuits or in both. The function of these shunt and series impedances is to compensate for variations in current ratio and phase displacement resulting from current changes, as more fully explained in my application, Serial Number 433,921. The secondary winding 17 has taps 18, 19 and 20. Connected to the secondary are different loads or burdens 21, 22,23 and 24. The burdens may be connected to one set of taps or to different taps if desirer.

In simplest form, the various burdens which may consist of watt-hour meters, ammeters, indicating ammeters, relays or other devices are connected to the leads similar to that shown by the burdens 21, 22 and 23. If only one burden, such as 21, is connected to the secondary, the ratio and phase angle of the current in the secondary burden 21 may be quite different from that which would flow if additional burdens 22 and 23 are connected. If, however, the possible output of the secondary of the current transformer is ample to supply the several burdens 21, 22, 23 and 24, compensating or dummy burdens, represented by resistance 25, inductive reactance 26 and capacity reactance 27, may be substituted for any or al of the various members 22, 23 and 24 when they are not connected to the system, and thus the conditions in the secondary will be maintained uniform at all times.

If, then, it is desired to connect a burden, such as 22 or 23, to the secondary, all that is necessary is to remove an equivalent portion of the dummy burden supplied by the various members 25, 26 and 27. This would keep the burden the same on the secondary. Any reduction or removal of the useful burden may be offset by the shunt burdens 25, 26 and 27. The phase angle of the current in the secondary with respectto that in the primary lead 10 may be controlled either by controlling the current in the shunt 13 or by impedance 28 it; the secondary or by the shunts 25, 26 and 2 If the total burden remains the same when individual burdens are introduced into or removed from the secondary, the phase angle of the current and voltage induced in the secondary will remain the same. Therefore, any meter or other device connected to the secondary will not be disturbed with the addition or removal of other burdens; it being understood that any change in load due to the removal or addition of apparatus will be compensated for by the shunt or series impedance members.

In general it may be advisable to control the phase angle for one of the secondary burdens, say 21, such as that for a watt-hour meter, power factor meter or diilerential relay. In this case the phase angle is adjusted to give the proper relation with all of the burdens connected to the secondary or with an equivalent compensating burden. It is evident that if the same burden is maintained by adjusting the compensating burden for any change in the individual burdens, the characteristics will not be changed insofar as any particular burden is concerned.

For example, if a watt-meter is placed in the circuit, the compensating load will have to be reduced by an amount equal to that added by the introduced meter, both as to magnitude and power factor or phase angle, and when this is done the characteristics of the complete secondary circuit, as well as the conditions of operation of each individual burden, will remain unchanged. If, on the other hand, a burden is removed from the secondary, the compensating burden must increase the burden by an equivalent amount, both as to current and power factor or phase angle. It, of course, is possible in some cases to apply the compensation across a portion of the secondary winding only as between the taps 19 and 20 or 18 and 19, as shown in my previous application. In general, however, it would be much more simple to place the compensating load across the same leads with the various useful burdens.

In place of the single set of compensating burdens 25, 26 and 27 the arrangement shown in Fig. 2 may be used in which burdens 29, 30, etc. are used to replace individual useful burdens, some of which are shown in place at 31, 32, 33, 34 and 34'. The burdens 29 and 30 may be removed all or in part depending upon the burdens inserted in their place. In some cases it may be more economical to adapt a burden to the voltage of the secondary by interposing a transformer as in the case of burden 34 than it would be to use appropriate impedances. It will, of course be understood that the burdens, such as 29 or 30, may be appropriately calibrated so that the addition or change in the useful burden may be compensated for by simply adjusting these compensating or dummy burdens. Dummy burdens similar to 29 and 30 may be made up so as to have similar characteristics to burdens which will be added later; the dummy burdens corresponding to the burdens of useful apparatus. If the burdens of useful apparatus are so connected with series or shunt impedances or a combination of both that their burdens can be made all uniform, the addition or removal of burdens at a later date may be greatly simplified.

By the use of appropriate impedance either in series or in shunt with the burdens 31, 32, 33 and 34, the total burden on the secondary may be composed of separate units, each of which complies with a given standard of resistance and reactance or impedance. The compensating or dummv units 29 and 30 will also be made up to conform to this standard so that any unit of the useful burden may be replaced by a standard compensating unit, thus greatly simplifying the operation of introducing or removing units in the circuit without affecting the other units or the entire set-up or group.

The burdens may be arranged in series, as shown in Fig. 3, in which the useful burdens are shown at 35, 36 and 37 These burdens may be standardized by suitable impedances 38 and'39 and substitute burdens 40 arranged to be switched in in place of any burden to be removed. Of course, it is not essential that all the useful burden units be brought to the same standard so long as the compensating units 40 each corresponds to the useful unit it replaces. Any burden as 35 may be connected to the circuit by a transformer 41 when this is found desirable.

A combination of parallel and series arrangements may be used, as shown in Fig. 4, in which the burdens 41 and 42 are arranged in series with substitute burdens 43 and 44 arranged to be connected by switches 45. The switches 45 are such that the substituted circuit is connected before the old circuit is broken. to maintain the continuity of the circuit. Parallel burdens 46, 47, 48 and 49 are shown, 46 and 47 being dummies and 48 and 49 useful.

Fig. 5 is a vector diagram in which the various duties 21, 22 and 23 are represented by vectors 21, 22 and 23' respectively. Since the currents are in multiple the resultant current 50 in direction and magnitude will be obtained by adding the several vectors. The burden on the secondary corresponding to the burdens 21, 22 and 23 must be the equivalent of 50 Where it is made up partly or completely by the corrective burden. If the secondary of the transformer and the corrective burden is such that a current 51 is of suflicient magni tude to take care of the reactive component and the vector 52 is suificient to take care of the unit power factor component at the same time, any number of burdens may be added providing the resultant does not extend beyond the sides of the parallelogram, of which 51 and 52 are two sides. y I

The method in general consists in providing a current transformer with a secondary having a sufficiently high output to furnish the necessary power for the maximum load and compensating for any burden added or removed so as to keep the burden constant and give the resultant current or potential developed by the secondary the desired relation to that of the primary current. If, in place of various burdens, the compensating burden is changed, it is possible to obtain difi'erent ratios without changing the relation of the number of turns between primary and secondary.

Where the current requirements of the primary may vary widely, it is possible to change the number of turns in the primary or the shunt impedance on the primary, or the relation between the series and shunt impedances for the primary. These changes will change the ratio without changing the number of turns. It is also possible to change the ratio by changing either'the shunt or series impedance in the secondary. If desired, several taps or windings may be provided in the secondary so that the effective number of turns may be changed to provide a different working range or current for the primary of the bushmg.

It is evident that the above method may be applied to other types of current transformers where the output and characteristics of the transformer is sufficient to supply the various burdens. The method, however, is more particularly applicable to high voltage current transformers where the characteristics of the ordinary current transformer are poor or where a wound type of current transformer is very costly, owing to the high insulation required between primary and secondary.

In low voltage work, it is customary to provide individual current transformers for the burdens. In high voltage work, however, where the ring type of current transformer is placed over the outside of the bushing, it is evident that the application of several of these ring type transformers which would be necessary to take care of several burdens would either seriously reduce the insulation between the live terminal of the bushing and the flange at the center or an additional length in the bushing would be required. The ring type of high voltage current transformer also takes up valuable space in the circuit breaker or transformer case which may not only necesstate larger bushings but larger rings as well. The internal type of current transformer with its high output, therefore, makes it possible to use a circuit breaker or transformer tank of minimum size.

" breaker operation, the

In the case of CllClllo current transformer is protected from hot gases and cracked oil or conducting material which may affect the insulation. The internal type of current transformer also has the advantage that it protected from the force of oil or gases following the operation of the circuit breaker. It is also protected from high voltage due to a conducting path of gas from one of the live terminals which may be possible with a ring type of current transformer unless the lat e is protected by a grounded pocket. The of the pocket not only eliminates a portion of tie insulation of the bushing which might be used to advantage out frequently sets up an unfavorabie electrostatic liold. The reduction in effective insulation of the bushing or in the clearance of the ring may result in an effective lowering of the inductive capacit o t tl.e

circuit breaker or voltage carrying pos, l ities of the device. In the ring type there If danger that radio disturban -e may be set up due to the potential difference between the bushing and the ring type transformer. While this may be prevented by a shield. the shield reduces the effective length of the bushing.

Where the same primary current passes through the primary of two current transformers, the secondaries of ti :se current transformers may be used in multiple series to supply the energy for the various burdens, permitting the attachment of the same burdens or dummy impedanccs as though the energy were supplied by a single secondary.

The invention greatly simplifies the instal lation of the control circuits as a single pai of le ds only is necessary between the secreformer and the ondar of the current tr switchbcarc. or control equipment. The same pair of leads may also be used to attach equipment rent points. il herc separate seconna ies 'e used to operate difle burdens, adt l leads must be us f wl increase the cost of installation. plicit, and economy so effected .lrw verv appreciable, particularlj. in la.ge installations.

tae proper arrangement of burdens or loads having difierent power factors, it possible to conserve the output of the secondary of the current transformer and. the same time. maintain the desired phase relation of the current in the different bun dens and also maintain the ame relation of current both as to magnitude and phase angle for the other burdens. Fig. 6 shows one arrangement for accomplishing this. It is assumed that the proper ratio has been established in the leads and 56 by any of the methods previously described.

Fig. 7 shows the phase angle of currents in difierent pieces of equipment where they are attached individually to a current transformer of proper ratio. 'Each instrument is assumed to take live amperes at normal full load. It will be noted that the resultant of all of these currents will be represented by K in magnitude and direction. Two currents E and F are shown in phase with each other as mi ht be the case of an indicating wattmeter and a power factor meter. It will also be noted that the currents represented by D, E, F and G are considerably in advance of the resultant current K and the currents A, B, C and H are lagging. lVhere several instruments are to be used together on the same transformer, those having a leading characteristic with respect to resultant K may be combined with those having a lagging characteristic as regard to resultant K, so that the result of the two may not vary greatly from that of K. lVhile the resultant of any two currents might not coincide with K, the fact that the resultant closely approximates K will simplify the problem of bringing the combined current in phase. In Fig. (3, E and C designate instruments having characteristics represented by vectors E and C in F ig. 7 and are shown in multiple with suitable impedances and shunts to regulate the magnitude of the current and maintain the proper phase angle for the current in each load or burden. Any difference in phase angle between the resultant current flowing through E and C from that of K may be adjusted either by the proper shunt impedances for the individual loads or by a shuntimpedance 57. Other loads or burdens such as H and D may be connected in a similar way when provided with suitable shunts.

Vhile it is generally assumed that the currents in all instruments or loads will be the same for full load conditions, this relation need not apply, and adjustments may be made upon any other basis. It is readily seen, however, that where burdens are properly combined to obtain the desired result that the current carried in shunts can be reduced. The same principle may also be applied in order to absorb as little as possible of the voltage out-put of the secondary of the current transformer.

In the arrangement shown in Fig. 8 the object is to make the out-put of the secondary of the current transformer as effective as possible for the operation of the useful burden and still maintain the proper ratio and phase relations for the current in the several burdens. In order to accomplish this result, it is necessary to reduce the portion of the out-put of the secondary of the current transformer that is absorbed in series or shunt impedances. The burdens D, E, F and G having approximately the same phase arrangement for current are arranged in series, and the burdens A, B, C, H are arranged in another series group. The various burdens are equipped with suitable shunt impedances Where necessary; they may also be equipped with series impedances although this may be applied at any point in the group, such as at 60 or 61. It will be seen that the function of the shunt impedances, represented by 62, is to give the current the proper phase relation in the various burdens irrespective of the resultant current flowing through the leads 60 or 61.

It is evident that in any series group the current in each burden, together with its particular shunt, must be the same in magnitude and phase angle as that for any other burden together with its shunt in the same series. One object of the shunt, therefore, about the individual burdens is to allow a different phase angle for the current in the burden from that of the resultant of the current in the burden and shunt. Where the proper relation is maintained, burdens in a particular series may be added or taken from the group by the substitution of dummy burdens or adjustments in the impedances, such as 60 or 61 without disturbing the relation of any of the other burdens. Where it is desired to attach a burden which requires more current or voltage, such as a tripping coil or solenoid 63, this may be attached in series with other groups used in multiple. Several members may be used in series with a greater or less number and another chain used in multiple, depending upon the characteristics of the various burdens and the secondary of the current transformer.

Where large currents are required for the operation of equipment located at a distance from the secondary of the current transformer, a step-up transformer 64, Fig. 9, may be used at the current transformer, connected to a step-down transformer 65 through leads 66. These transformers may be provided with suitable taps for varying the ratio. By suitable transformation the current in the leads 66 can be so reduced that the I. R. drop in the leads will be very small compared to that where the current is run directly from the secondary to the burden.

The arrangements shown have the advan tage that burdens may be added to current transformers or removed without disturbing any desired relation in other burdens. Where the available supply to the burdens is limited, either through limitations in the current transformer or in the leads, the invention provides for utilizing the output of the current transformer to best advantage. While the invention may be used for any type of current transformer, it applies more particularly to transformers having a high output such, for instance, as that shown in my prior Patent No. 1,723,000. The multiple arrangements make it possible to keep down the turn ratio of primary to secondary in the transformer which has a material advantage in construction. I

Where the number of turns in the secondary is small, the. internal reactance is less. This permits placing the necessary adjusting shunt or series impedances in the external circuit to obtain the required ratio for the active or useful burdens. The possible arrangement of the burdens, together with the series or shunt impedances, apply to the transformer and also makes it possible to changev the characteristics of the current transformer very materially. This may be important for relay operation as the ratio may be controlled over a much wider range. By changing the burden, either useful or adjusting, the ampere turns in the secondary may be changed materially. As the current in the secondary is in opposition to the current in the primary, it is seen that an increase in thecurrent in the secondary will cause a de-magnetizing effect upon the magnetic core, therefore, saturation of the core will not take place until a higher current value is reached in the primary. From this it will be seen that the arrangement of the burdens is important in changing the characteristics of the current transformer as applied to the useful burdens.

Apparatus or burdens may be connected in a number of different ways or combinations to conserve the output of the current transformer, to change the general characteristics of the transformer, to match other current transformers or. to obtain a desired phase angle for some of the burdens.

In the arrangement shown in Fig. 10 a current transformer having sufficient output in the secondary is used to supply the current for a number of burdens, part of which are used in multiple and part in series. In this arrangement the primary 11 is provided with suitable shunts or impedances, as explained in connection with Fig. 1, and the secondary 17 is designed to give the necessary output for the operation of the various burdens. Shunt impedances 70 and a series impedance 71 may be used if desired to control the ratio or phase angle of the current supplied to the secondary burdens. With these adjusting impedances, it is not only possible to correct ratio and phase angle, but adjustment may be made for the addition or removal of burdens to the secondary circuit.

Where the output of the secondary of the current transformer is sufficient to operate not only the useful burdens but also addibe obtained by using a series impedance which will tend to permit an early saturation of the core. On the other hand, if it is desired to delay the saturation of the core, proper adjustment of the core for the secondary burdens may be obtained by the use of a shunt impedance. This tends to increase the die-magnetizing ampere turns affecting the core of the current transformer Which delays saturation.

By using proper combinations of series and shunt impedances, the desired ratio cannotonly be obtained but th characteristics of the current transformer can be modified as vell. This may be of material benefit in matching the characteristics of other current transformers or for protecting instruments and connected apparatus against overloads. It is evident that the adjusting impedance may be used to correct ratio as well as phase angle, the method being disclosed in my prior application Serial Number $3,921. The impedances may consist of reactances having either air or magnetic cores, resistances or condensers, either singly or in any desired combination.

If the desired current and ratio is obtained for the secondary burdens by the insert-ion of a suitable series impedance as 71, the demagnetizing ampere turns in the secondary circuit will be less than where the proper adjustment is obtained by a shunt impedance as 70. In the shunt impedance, the same voltage may be applied to the useful burdens as in the case where the series impedance is used but in the case of the shuntimpedance, the saturation of the core of the current transformer is delayed, due to the greater current in the secondary of the transformer and consequent increase in ale-magnetizing ampere turns. From this it naturally follows that by the proper regulation of shunt and series impedance, it is possible to materially change the characteristics of a given current transformer without necessarily changing the number of turns in either pri-. mary or secondary, thereby making it possible to use a given transformer over a much. wider range than was heretofore possible.

The shunt impedances may be placed on the transformer side of the series impedance 71 or on the burden side, providing the necessary adjustments are made. In general, however, the shunt impedances will be so placed that the energy out-put of the current transformer will be conserved, although the (lo-magnetizing effect may be increased.

In Fig. 10 relay 73 is shown directly across the secondary circuit without adjusting impedance, the voltage in the secondary circuit being adjusted to obtain the proper currentfor this relay. A graphic ammeter, having a somewhat different character, is shown at T l. The current in 74 is adjusted by suitable impedances placed in series, as the flow of current would be entirely too high for this instrument because its impedance is less than that of the relay shown at 73. In the particular example illustrated in Fig. 10, the voltage applied to the relay 73 is 3.4 volts. Since the total voltage for the indicating watt-meter 75, indicating ammeter 7 6, the Watt-hour meter 77, the graphic wattmetcr 78 and the power factor meter 79 is less than 3.4 volts, they may be connected in series and an impedance 80 placed in series. This impedance will not only serve to provide the proper current in the various burdens but can be used to correct the phase angle of the current for some one of the burdens, Where it is desired to maintain the proper phase angle, such as in the case of an indicating wattmeter.

The vector diagram showing currents and voltages for the series burdens is shown in Fig. 11. The currents for the individual groups are shown vectorial-ly in Fig. 12 as Well as the resultant current. The voltage diagram for the series group is shown in Fig. 13. It is evident that any combination of series or multiple burdens may be used, depending upon the characteristics of the current transformer and the amount and character of the load imposed by the burdens.

lVhere the burdens are to be applied at a considerable distance from the secondary of the current transformer, it may be advisable to develop a high voltage 'ather than a large current in the secondary of the current transformer or to transform the current by suitable ratio transformers or by auto transformers. This permits the use of small leads between the current transformer and the burdens. Since the current required for the series burdens will, in general, be 5 amperes only for normal full load conditions, the series burdens may be placed at a distance and the shunt load such as trip coils and other equipment placed near the current transformer. The series and multiple groupings make it possible to reduce the number of leads from the current transformer which is a material advantage over that Where separate current transformers are used.

Where it is desirable to use compensating impedances either in shunt or in series, the secondary of the current transformer must not only develop sufficient. voltage to energize properly the various burdens but the transformation ratio of primary current to secondary current must be sufiiciently low so that the necessary current may be developed in the secondary circuit or burdens. If the ratio, without usting impedances, is that normally desired for the burden, it is evident that the use of an impedance will reduce the current in the burden so that the proper indication of current will not be obtained for normal full load current in the primary. If, on the other hand, the nominal ratio of the current transformer is suiiiciently low so that the current in the secondary will be higher than that desired for the full load condition, the proper impedances may be used so that the desired current and ratio for the secondary burdens will be obtained.

If the volt-ampere output ofthe secondary is suflicient but the current ratio is too small, it is possible to use another current transformer to supply a larger current at a lower voltage for the secondary burden. In general however, it is advisable to use a lower ratio than that desired for the useful burden as this permits compensation for changes in the burden for either current voltage or phase angle.

Where the transformer in the secondary is wound for a lower ratio than that for the burden, the number of turns is reduced which may be of material advantage in the cost and construction of the secondary of the current transformer. The internal impedance is also reduced where the ratio is lowered. This method of construction and operation permits the use of a given current transformer over a much wider range of ratio without q change in the turn ratio between secondary and primary.

While the above means of construction and operation may be applied to any type of current transformer, the construction and operation is particularly applicable to current transformers used in connection with high voltages where it is difficult to obtain the necessary insulation and output of the current transformers as normally made. Changes in the turn ratio for current transformers applied to very high voltages are difiicult to make. IVhere the output, however, of the secondary is suflicient to permit compensating impedances, a few turn ratios will permit a current transformer to be used for practically any range of current in the primary without the necessity of producing a complicated and costly transformer.

In Fig.14 an arrangement of apparatus is shown which may be very desirable for some installations. The series burdens are placed in multiple with burdens 101 and 102. The current used for the burdens 101 and 102 and the series group is caused to pass through a solenoid or trip coil 103. This method makes it possible to supply a large actuating force for the solenoid or trip coil 103. -Where the coil is provided with a movable core or an armature 104, it is evident that the closing of the armature will permit an increase in the lines of force through the coil and a corresponding increase in the impedance of the device. This maybe used .to advantage to safeguard instruments where the current will be too high. In cases, however, where it is desired to disturb the normal relation of a secondary circuit little, if any, the increase in the impedance in the solenoid or trip coil can be offset by changing the effective ampere turns in the coil or by applying asuitable shunt impedance. This can usually be brought about without seriously affecting the operation of the coil as the torque increases veryrapidly with the move ment of the armature or core.

The reduction in the effective ampere turns or magnetizing force may be brought about in several different ways, such as regulating the resistance in a shunt turn or by changing the number of turns; the change being eifected by the armature 104 in any suitable way. WVhere only a single adjustment is necessary, this may be used to close a contact 105 for the shunt. It is evident that the change in impedance, due to movement of the core or armature, may be used for correcting means.

In Fig. 15 the vector 100 is the vectorial sum of the voltage on the series burdens 100 and the vector 100 represents the current in the series burdens. In Fig. 16, 101, 102 and 103 represent the currents flowing in the parallel circuits, respectively, of Fig. 14, and represents the vectorial sum or the total current.

In Fig. 17, 111 represents the voltage on the parallel circuits of Fig. 14, 112 represents the voltage drop over the relay 103, and 113 is the total voltageon the circuit while 110 represents the total current.

In Fig. 10 a relay 106 is connected in shunt across the secondary and the change in characteristics of the coil or solenoid of the relay is used to offset the effect of the saturation of the core of the current transformer. When this occurs, more current will be available for the remaining burdens, thereby tending to maintain the proper ratio for these burdens over a wider range of current in the primary. By using several impedances, it is possible to obtain practically any desired adjustment or correction. Then the current in the primary of the current transformer becomes great enough to begin to saturate the core, the armature of the solenoid will be raised, thus increasing the impedance of the solenoid and consequently of the shunt of which it is a part.

I claim:

1. The method of operating a current transformer comprising the steps of adjusting the relative values of the current in the primary and secondary of the transformer for different currents in the primary to compensate for variations in transformation ratio and phase angle and maintaining a substantially constant burden on the secondary to prevent variations in transformation ratio and phase angle due to changes in the secondary burden.

2. The method of operating a current transformer having a variable compensating burden in the secondary thereof comprising the steps of compensating for changes in cur rent in the primary of the transformer to maintain a substantially constant transformation ratio and phase relation between the current in the primary and secondary circuits and adjusting the compensating burden in the secondary to maintain a substantially constant burden on the secondary of the transformer when the useful burden on the secondary is changed.

3. The method of operating a current transformer in which a plurality of electrical translatin devices having other than unity power factors are connected in the secondary of the transformer comprising the steps of removing some of said translating devices and substituting a compensating burden equivalent both in magnitude and power factor to that removed for each translating device removed from the secondary to maintain a burden in the secondary of substantially constant amount and character.

4. A current transformer in which a plurality of electrical translating devices differing from one another in magnitude and/or character are connected in the circuit of the secondary of said transformer and each translating device has associated therewith, where necessary, sufficient impedance to constitute a unit burden of a predetermined standard magnitude and character so that the burden. of each translating device and its associated impedance is the same in amount and character as every other translating device and its impedance in the circuit.

5. The combination with a current transformer, of a burden in the secondary of said transformer comprising a plurality of units of uniform amount and character, some of said units including a translating device having a power factor other than unity, and others of said units forming compensating burdens to maintain a substantially constant total burden on the secondary irrespective of changes in the useful portion of the burden.

6. The method of operating a current transformer comprising the steps of adjusting the relative current in the primary and secondary of said transformer to maintain a substantially constant ratio for varying loads on the primary of said transformer and maintaining a substantially constant burden on the secondary of said transformer.

7. The method of operating a current transformer having a useful and a compensating burden in the secondary, said method comprising the steps of compensating for variations in the phase angle and transformation ratio due to the change in the load on the primary of said transformer and adjusting the variable compensating burden on the secondary of said transformer for maintaining a substantially constant total burden on the secondary of said transformer when changes are made in the useful burden on said transformer.

8. A current transformer having a plurality of burdens in the secondary of the transformer, and impedances in the circuits of the individual burdens where necessary to maintain predetermined operating conditions for the various burdens irrespective of the effect of the other burdens upon the total burden on the secondary of the transformer.

9. A current transformer having a plurality of burdens in the secondary of the transformer, including compensating burdens for maintaining the total burden on the secondary constant in amount and character, and impedances in circuit with the individual burdens where necessary to maintain the desired operating conditions for the individual burdens notwithstanding the total burden on the secondary.

10. The method of operating a current transformer having a useful burden in the secondary, and a variable impedance in shunt with the useful burden said method comprising the step of increasing the said variable impedance as the current in the primary increases and the core of the transformer approaches saturation, to divert a larger portion of the secondary current to the useful burden and thus maintain a more nearly constant ratio between the current in the useful burden and the current in the primary of the transformer.

11. A currenttransformer having a useful burden in the secondary thereof and a relay in shunt With said useful burden, the relay being adjusted to operate and thus increase the impedance thereof when the current in the primary of the transformer increases to a predetermined value and the magnetic core of the current transformer approaches saturation, the increase in the impedance of the relay causing a larger proportion of the secondary current to be diverted from the relay shunt to the useful burden, thus main taining a more nearly constant ratio between the current in the useful burden and the current in the primary of the transformer.

12. The method of operating a current transformer having a useful burden in the secondary circuit thereof and a relay in series with said circuit, said method comprising the steps of adjusting the impedance of the relay circuit to offset the effect of increase in the impedance of said relay due to operation of said relay.

13. The method of operating a current transformer having a useful burden in the secondary circuit thereof, and a relay in said circuit said method comprising the steps of shunting a portion or all of the winding of said relay when said relay operates to offset the increase in the impedance of said relay due to decrease in the reluctance of the magnetic field of the relay on operation of the relay.

14. A current transformer having a lower ratio of secondary to primary than is necessary to supply the required current for the useful burden on the secondary, an impedance being connected in circuit with the secondary to reduce the current in the useful burden to the requisite amount and to permit adjustments.

15. A current transformer in which the transformer is provided with a lower transformation ratio of secondary to primary than is necessary to supply the requisite current to the useful burden in the secondary, said transformer having a plurality of burdens in the secondary and impedances in circuit with the burdens to adjust the current and phase angle to suit the various burdens.

wherein a single transformer supplies energy to aplurality of useful burdens so that the current in the secondary winding of the transformer will be sufiicient to materially oppose the magnetizing effect of the primary current and delay saturation of the magnetic core of the transformer.

17. The method of operating a current transformer having useful burdens in the secondary and series and shunt impedances connected in circuit with the useful burdens in the secondary for controlling the current in the useful burdens, said method comprising the steps of controlling the saturation point of the magnetic core of the transformer by adjusting the series and shunt impedances.

18. The method of operating a current transformer in which a plurality of useful burdens are connected in the secondary circuit of the transformer, and impedances are connected in circuit with the useful burdens to control the amount and character of energy supplied to the useful burdens, the current transformer being wound to supply the maXimum energy required by the total burden on the secondary, said method comprising the step of adjusting the shunt and series impedances in the secondary to control the amount of current in the secondarry and regulate the magnetic flux in the core of the current transformer for various primary currents.

19. A current transformer having a plurality of burden units connected in the secondary thereof comprising groups formed of electrical translating devices and compensatin g impedances differing from one another in power factor but grouped so that the power factor for each group is substantially 16. A current transformer arrangement 

